Abstract: Timing, Character, and Preservability of Sedimentation Events in Low-Energy Coastal Environments, Southwest Florida

H. R. Wanless, J. M. Rine

Shallow mangrove-rimmed bays and channels in the Marco Island area, southwest Florida, accumulate sediment during both prevailing low-energy conditions and storms. The timing and resultant record of this depositional system are a striking contrast to the more open bays and platforms of subtropical south Florida.

Sediments in protected coastal environments of southwest Florida consist of quartz and skeletal-carbonate sand and silt and clay-size mangrove detritus, diatom frustules, fragmented carbonate particles, and organic debris. The vertical sedimentary sequences are organized into vague to distinct alternations of mud layers (10 to 50 cm thick) and quartz-sand and shell-fragment layers (1 to 10 cm thick). Sand layers result from substrate winnowing and offshore-sediment influx during storms. Mud layers accumulate during prevailing low-energy conditions and poststorm settleout. Mud is derived from persistent release of decayed mangrove detritus from coastal swamps, settleout of planktonic microorganism remains and, locally, benthic microorganism production. Layering is well to vaguely prese ved because, (a) mud accumulation between storms buries sand layers below the focus of bioturbation and, (b) prevailing high turbidity reduces burrowing and grazing communities in deeper areas.

This sedimentary process and product contrasts sharply with the more open bays and platforms of south Florida in which prevailing low-energy conditions are periods of benthic skeletal production and intense biogenic reworking. Sediment-dispersal and redistribution occur only during winter storms and hurricanes. Surface-storm mud layers (1 to 20 cm thick) or winnowed shell lags are bioturbated quickly leaving a homogenized sedimentary record.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA